Denim is a woven cotton cloth wherein the warp thread has been dyed, usually blue, with the dye, indigo. One desirable characteristic of indigo-dyed denim cloth is the look created by the alternating blue and white threads of the warp and weft yarns, which upon normal wear and tear gives denim a white on blue appearance. A popular look for denim is the stonewashed look. This stonewashed look consists of a generally lighter blue color than unwashed denim with localized areas, particularly around the seams, of even lighter color. Stonewashed material often has a softer texture and maintains the desirable white on blue contrast. Traditionally, stonewashing has been performed by laundering the denim material in the presence of pumice stone, which results in fabric having a faded or worn appearance and the desired white on blue contrast described above.
Enzymes, particularly cellulases, are currently used in processing denim. In particular, cellulases are used to give denim a stonewashed appearance without the need for as high a loading of the pumice stones that are used in traditional stonewashing. This processing method is referred to herein as enzymatic "stonewashing", even if no stones are present in the washer. Use of enzymes to stonewash has become increasingly popular because use of stones has several disadvantages. For example, stones used in the process cause wear and tear on the machinery, environmental waste problems due to the grit produced and result in high labor costs associated with the manual removal of the stones from the machines and the pockets of garments. Consequently, reduction or elimination of stones in the wash may be desirable.
Contrary to the use of pumice stones, enzymes (particularly cellulases) are safe for the machine, result in little or no waste problem and drastically reduce labor costs. Therefore, it may be beneficial to use enzymes for stonewashing. However, even though the use of enzymes such as cellulase may be beneficial as compared to stones, there are some problems associated with the use of enzymes for this purpose. For example, one problem with some cellulases, such as cellulases from the wood-rotting fungus Trichoderma, is what could be described as a "redeposition" or "backstaining" (both terms used interchangeably herein) of some of the dye back onto the fabric during the enzymatic stonewashing process. Such redeposition or backstaining leads to a blue coloration on the white denim threads, resulting in less contrast between the blue and white threads and abrasion points (i.e., a blue on blue look rather than the preferred white on blue). See American Dyestuff Reporter, Sept. 1990, pp. 24-28.
Redeposition or backstaining is objectionable to some users. For example, even though Trichoderma cellulases exhibit a much higher specific activity on denim material than Humicola cellulases, cellulases from Humicola are often preferred because of their lower level of backstaining. This is so, even though the much higher potency of Trichoderma cellulase permits the use of smaller quantities of enzyme to achieve a higher degree of abrasion in significantly shorter processing times.
The problem of redeposition of dye during stonewashing has been a concern of denim processors. Previous attempts to address the problem with Trichodermna cellulase compositions include addition of extra anti-redeposition chemicals, such as surfactants or other agents, into the cellulase wash to help disperse the loosened indigo dye and reduce redeposition. In addition, denim processors have tried using cellulases with less specific activity on denim, along with extra rinsings. This results in additional chemical costs and longer processing times. Another method attempting to address the redeposition problem includes adding a mild bleaching agent or stain removing agent in the process. This method affects the final shade of the garment and increases processing time.
While these methods aid to some limited degree in the reduction of redeposition, the methods are not entirely satisfactory and some objectionable backstaining remains. Use of enzymes and stones together may be advantageous in decreasing the degree of redeposition; however, it leaves the processor with some of the problems associated with the use of stones alone.
Another method, as described by Clarkson et al in PCT Publication No. WO 94/29426 (hereafter "Clarkson et al"), has been to include an added protease enzyme in the stone washing treatment. It was found that the treatment of denim with a composition comprising a redepositing cellulase and an added protease improves the contrast between white and blue threads and reduces dye redeposition. Acting in the washing machine, the proteases are somehow thought to prevent the cellulase proteins from binding the colored particles back onto the surface of the denim, and yet, when used in moderation, they do not have a severe adverse effect on the resultant abraded look caused by the action of the cellulase. This method, while providing some advantages, is costly and requires careful control because proteases, by their very nature, tend to destroy cellulase enzymes. The practitioner must strike a balance between the desirable proteolytic effect on reducing backstaining and the undesirable proteolytic effect on reducing the activity of the cellulase.
In the process of Clarkson et al, the protease is used essentially as a stain remover or staining inhibitor and must be included in the washing process. Clarkson et al teach three options; (1) add the protease directly to the washer with the cellulase enzyme, (2) add the protease to the rinse cycle after a cellulase treatment, or (3) blend the protease with the cellulase prior to washing. Relative to the basic process of adding the protease and cellulase together, adding the protease to the rinse cycle avoids significant proteolytic attack on the cellulase, but it has the disadvantage of adding an extra processing step. Pre-blending the cellulose and protease, on the other hand, makes for a simple easy-to-use formulation, but results in a difficult balance between the desirable proteolytic effect on stain removal and the undesirable proteolytic effect on destroying cellulose activity. For example, a highly active protease may completely destroy the cellulase enzyme during the normal time it takes for storage and shipping. This shelf stability problem can be managed but requires; (1) selection of a protease that has good anti-staining power, but can digest cellulase to, at most a limited extent, (subtilisins, which are not highly active against cellulase but are well known as potent stain removers are a preferred choice), and (2) a pre-incubation of the selected protease and cellulase at an elevated temperature to ensure that what proteolytic attack there is on the cellulase is taken to completion and that a commercial formulation will be stable during storage and shipping.
The action of stronger proteases, particularly the protease papain on Trichoderma cellulases, has been investigated extensively. It has been found that limited amounts of papain digestion can split the core domains of Trichoderma cellobiohydrolases apart from their natural binding domains. This has the effect of essentially eliminating any measurable activity these enzymes have against crystalline cellulose such as Avicel or cotton while still preserving their activity against soluble substrates such as .beta.-glucan. As a result, prior workers concluded that the natural binding domain plays a critical role in enabling the cellobiohydrolase enzyme's attack on crystalline cellulose. The extent of treatment needed for papain to completely eliminate CBH activity on crystalline cellulose was roughly 0.1 to 0.5 grams of papain protein per gram of cellulase protein, all multiplied by the treatment time in minutes (g min/g), that is, weight ratio of protease protein to cellulase protein multiplied by the treatment time.
Based on the shortcomings of previously attempted methods for reducing or preventing redeposition, there is a need for more easily controlled and more cost effective methods to address the issue of redeposition or backstaining of dye during stonewash treatment.
Accordingly, it would be desirable to find an enzymatic composition or method that would be cost effective, have good shelf stability, high potency, and not include a redepositing or backstaining cellulase.